This invention relates to a device for controlling an optical pickup and, more particularly, to a device for controlling an optical pickup of one-beam system using a four-split photo-detector capable of eliminating deviation of a beam spot on the photo-detector due to aging of an objective lens support system or other causes and thereby positioning a neutral point of the beam spot constantly at the center of the photodetector.
An optical pickup of one-beam system using a four-split photo-detector is used for detection of an RF signal and various servo controls including focusing servo and servo controls in the radial direction (i.e., tracking servo and feed servo) in a playback device for playing back optical discs such as a Compact Disc for the Compact Disc Digital Audio System and a video disc.
As a prior art radial direction control circuit using a four-split photo-detector, a circuit using a heterodyne system for tracking error detection is shown in FIG. 2. In a four-split photo-detector 10 (PPD, i.e., pin photo-diode), signals representing received laser beam on respective diagonals are added together. Sum signals A+C and B+D are subject to subtraction by a subtraction circuit 12 to produce a signal (A+C) -(B+D). These sum signals are also added together by an addition circuit 14 to produce a signal (A+C)+(B+D).
The output of the subtraction circuit 12 is zero when there is no tracking error and, when there is a tracking error, precedes or delays in phase by 90 degrees with respect to the output of the addition circuit 14, i.e., RF output, in accordance with the direction of the error (the level of the output of the subtraction circuit 12 also changes in accordance with the magnitude of the tracking error). A zero-crossing detection circuit 16 detects zero-crossing of the output of the addition circuit 14. At zero-crossing in rising of the output of the addition circuit 14, the circuit 16 momently turns on a switch Sa among switches Sa and Sb to which the output of the subtraction circuit 12 is provided thereby causing the output of the subtraction circuit 12 at that moment to be sampled at a capacitor Ca. At zero-crossing in falling of the output of the addition circuit 14, the zero-crossing detection circuit 16 momently turns on the switch Sb thereby causing the output of the subtraction circuit 12 at that moment to be sampled at a capacitor Cb. Since, as described above, the output of the subtraction circuit 12 precedes or delays in its phase by 90 degrees with respect to the ouput of the addition circuit 14 in accordance with the direction of tracking error, polarities of the sampled values in the capacitors Ca and Cb are of opposite polarities to each other. The sampled values in the capacitors Ca and Cb are applied respectively to a input and a+ input of a differential amplifier 18 and, accordingly, a tracking error signal of a level corresponding to the magnitude of the tracking error and of a polarity corresponding to the tracking error direction is provided from the differential amplifier 18.
The tracking error signal is supplied to a tracking actuator 22 through a phase compensation circuit 20 to drive the objective lens in the radial direction of the disc. The tracking error signal is also supplied to a phase-compensation and low-pass filter circuit 24 in which a low frequency component, i.e., a dc component, of the tracking error signal is extracted. This dc component is supplied to a feed motor 26 to drive the entire optical pickup in the radial direction of the disc so as to reduce the dc component to zero.
The objective lens of the optical pickup is adjusted in the manufacturing process in such a manner that, when the tracking servo is not on, i.e., no drive voltage is applied to the tracking actuator 22 and the objective lens is positioned at a neutral position determined only by spring constant of the support system, the beam spot will be positioned at the center of the four split photo-detector 10 as shown in FIG. 3.
Due to change in the spring constant caused by aging, change in positions of optical component parts due to aging or inclined disposition of the disc playback device or the disc, the beam spot tends to be deviated from the center of the four split photodetector 10 as shown in FIG. 4 (this state will be called "off-center" hereinafter).
In the prior art circuit shown in FIG. 2, in which the feed motor 26 is controlled so that the dc component of the voltage applied to the tracking actuator 22 will be reduced to zero, the tracking error affects the feed servo. If there is off-center, the beam spot tends to converge in the four split photo-detector in a deviated state by the amount of this off-center component. This off-center component is superposed on the tracking error signal and is applied for the tracking servo. Incidentally, the feed servo is operating so that the dc component of the voltage applied to the tracking actuator 22 becomes zero. At this time, the off-center itself is not corrected. Namely, the off-center component exists in the tracking error signal with resulting deterioration in the quality of the tracking error signal and also decrease in the tracking efficiency due to deterioration in the tracking error signal. The deterioration in the tracking efficiency causes failure in introducing the servo state or results in taking undue long time in introducing the servo state when the mode is changed from a tracking-off mode to a tracking-on mode, for example in starting of playback of the disc or during search.
It is, therefore, an object of the invention to provide an optical pickup control device capable of controlling the beam spot in such a manner that the offcenter is corrected and the beam spot comes at the center of the four split photo-detector so that the quality of the tracking error signal will be improved with resulting improvement in the tracking efficiency.